Actor Griffin Gluck is growing up and cashing checks ... scoring a sweet contract to play a high schooler on Netflix's new true crime satire, "American Vandal."

Griffin -- known for playing family son roles on movies like "Just Go With It" and "Why Him?" and the TV show "Private Practice" -- landed the series regular role of Sam on the upcoming mockumentary ... at a rate of $7,500 per episode.

Gluck's in all 8 eps ... so that comes out to $60k. Not bad for a 16-year-old.

It gets better -- if 'American Vandal' gets picked up for a second season, he gets a pay hike -- $10k an ep ... and $12,500 if there's a season 3.

You go, Griff.

The show premieres September 15.